High frequency diode having simultaneously formed high strength bonds with respect to a diamond heat sink and said diode

ABSTRACT

High frequency diodes are manufactured by methods forming an efficient heat path from the active diode junction through a diamond heat conducting member to a heat sink. A planar preformed element which becomes a permanent part of the diode structure is used to transfer the forces which form the bond between the diamond heat conducting member and the heat sink; simultaneously, the preformed element is bonded to an opposite side of the diamond, becoming a permanent part of the high frequency circuit of the diode.

United States Patent Potter 1 Mar. 18, 1975 1 HIGH FREQUENCY DIODEHAVING 3,702,975 11/1972 Miller 317/234 A SIMULTANEOUSLY FORMED HIGH3,761,783 9/1973 Kroger et al 317/234 A STRENGTH BONDS WITH RESPECT TO ADIAMOND HEAT SINK AND SAID DIODE Curtis N. Potter, l-lolliston, Mass.

Sperry Rand Corporation, New York, NY.

Filed: Sept. 13, 1973 Appl. N0.: 396,960

Inventor:

Assignee:

US. Cl 357/81, 357/56, 357/79, 29/589 Int. Cl. H0ll 3/00, I-IOll 5/00Field of Search 317/234, 1, 4, 6, 235, 3l7/47.l; 29/589 References CitedUNITED STATES PATENTS 7/1969 Moroney et a1 317/235 AK 12/1969 Gri et a1.317/235 AK 7/1972 Collard 317/234 A OTHER PUBLICATIONS Diamond as anInsulating Heat Sink for a Series Combination of IMPATT Diodes;Proceedings of the IEEE, Apr. 1968; pp. 762-763.

Primary Examiner-Andrew J. James Attorney, Agent, or Firm-Howard P.Terry 57 ABSTRACT High frequency diodes are manufactured by methodsforming an efficient heat path from the active diode junction through adiamond heat conducting member to a heat sink. A planar preformedelement which be comes a permanent part of the diode structure is usedto transfer the forces which form the bond between the diamond heatconducting member and the heat sink; simultaneously, the preformedelement is bonded to an opposite side of the diamond, becoming apermanent part of the high frequency circuit of the diode.

5 Claims, 2 Drawing Figures HIGH FREQUENCY DIODE HAVING SIMULTANEOUSLYFORMED HIGH STRENGTH BONDS WITH RESPECT TO A DIAMOND HEAT SINK AND SAIDDIODE BACKGROUND OF THE INVENTION 1. Field of the Invention Theinvention pertains to high power, high frequency or microwave diodes ofthe general type employed in transmission line amplifiers andoscillators and to methods of manufacture of such diodes. The inventionmore particularly relates to microwave diodes which operate at highpower, continuous wave or pulsed levels and which therefore requireeffective arrangements for removal of heat generated at their activejunctions.

2. Description of the Prior Art Generally, prior art high frequencydiodes expected to permit relatively high power operation in microwaveamplifiers or oscillators, such as high efficiency mode devices, havesuffered from various difficulties. Some of these are imposed by thenature of the high efficiency mode circuit devices themselves. Theselatter problems have been discussed in the generally availableliterature and in the M. I. Grace U.S. Pat. No. 3,646,581 for aSemiconductor Diode High Frequency Signal Generator," in the M. I. GraceU.S. Pat. No. 3,646,357 for a Semiconductor Diode High Frequency SignalGenerator, in the M. 1. Grace, H. Kroger, and H. .l. Pratt U.S. Pat. No.3,714,605 for a Broad Band High Efficiency Mode Energy Converter," andin other Sperry Rand Corporation patents and pending patent applicationson similar devices.

A primary direct limitation found in prior art high frequency diodes hasbeen connected with the need greatly to improve dissipation of heat fromthe active junctions of the diodes. While many successful attempts havebeen made in the past to fabricate circular and ring shaped diodes, lackof perfect forming of bonds to efficient heat sinks has generallyhindered effective heat removal from the diodes and has not permittedtheir reliably repeatable operation. Other very successful approaches tothe problem have involved the use of multiplicities of diodes along withenergy combining networks, such as are described in the U.S. Pat. No.3,605,034 to C. T. Rucker and J. W. Amoss for a Microwave NegativeResistance Transducer and in the U.S. Pat. No. 3,662,285 to C. T. Ruckcrfor a Microwave Transducer and Coupling Network, both patents beingassigned to the Sperry Rand Corporation. While valuable solutions to theproblem are thus afforded, the initial cost of such combining networksystems may be relatively high.

SUMMARY OF THE INVENTION The present invention relates to high power,high frequency or microwave diodes of the general type employed in highefficiency transmission line amplifiers and oscillators and to methodsof manufacture of such diodes. Such novel microwave diodes operate athigh continuous wave or pulsed power levels and therefore require highlyeffective arrangements for removal of heat generated at their activejunctions. Such high frequency diodes are manufactured according to thepresent invention by methods forming efficient heat paths from theactive diode junction through a diamond heat conducting member to acooperating heat sink. A planar preformed element which becomes apermanent part of the diode structure is used to transfer the forceswhich form the bond between the diamond heat conducting member and theheat sink; simultaneously, the preformed element is bonded to anopposite side of the diamond, becoming a permanent part of the highfrequency circuit of the diode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showingan elevation cross section of the invention.

FIG. 2 is an elevation view illustrating the method of bonding of partsof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel microwave or highfrequency diode structure is illustrated in FIG. 1 as being affixed to arelatively large heat sink 1, which heat sink may consist of a massivecopper plate or a plate of some other metal having similarly good heatconducting properties. As is seen in FIG. 1, the principal elements ofthe frequency diode structure include the semiconductor diode 11 mountedupon a heat conducting element 5 having very low thermal impedancecharacteristics, the latter being bonded in turn to heat sink 1.

The semiconductor diode 11 may, for example, be a trapped plasmaavalanche triggered transit diode, such as is generally known as theTRAPATT diode, and which finds application in high efficiency microwaveor high frequency amplifiers or oscillators of the general typedescribed in the M. I. Grace U.S. Pat. No. 3,605,004, issued Sept. I4,1971 for a High Efficiency Diode Signal Generator, and U.S. Pat. No.3,646.581, issued Feb. 29, 1972 for a Semiconductor Diode High-FrequencySignal Generaotr," both patents being assigned to the Sperry RandCorporation. Such avalanche diodes generally dissipate several times asmuch power in the form of heat as is usefully converted into microwavepower. However, high current densities must be obtained for highefficiency operation of the avalanche diode, but this may be reliablyachieved only if heating of the diode junction is minimized. Efficientremoval of heat from the diode junction allows the device to be operatedat higher input power levels, which consequently allows higher powergeneration with improved conversion effeciency.

The need of providing a good heat sink for an avalanching transit timediode is satisfied according to the invention by use of a certain typeof diamond found to have thermal conductivities about five times that ofcopper at room temperature (300 K. While the thermal conductivity ofsuch diamond material falls off inversely with increasing temperature,it is still twice as good as copper at elevated temperatures (500 K.).Type Ila diamonds are found to exhibit the highest thermal conductivityof any available material.

The heat conducting element 4 is therefore preferably composed of suchdiamond material and has opposite sides 4a adn 4b which have beenpolished and are generally parallel, while other sides such as side 5 ofthe diamond, for reasons of economy, remiain roughly cut and irregular.The diamond heat conducting element 4 is prepared for use by applicationto its opposite polished sides of very thin layers 3 and 6 of chromiumhaving typically a thickness of about 300 Angstrom units. The respectivechromium layers 3 and 6 are each coated, in turn, with a thin layer ofgold to a depth of about L000 Angstrom units. The gold layer 2 is bondedto heat sink 1-, while the gold layer 7 is used to form a bond with thediode ll, as will be described.

The diamond material is prepared for receiving the chromium and goldlayers by mechanically polishing the two opposedsurfaces 4a and 4b,which surfaces are then prepared for the chromium deposition, forexample, by washing in hot sulfuric or chromic acid, followed by asuccession of rinses with pure water and final drying. THe chromiumlayers 3 and 6 are then applied by evaporation to the required depth.The gold layers 2 and 7 may be formed next also by evaporation, and aremade about 1,000 Angstrom units thick, being firmly bonded respectivelyto chromium layers 3 and 6. It is found desirable that the chromiumlayers 3 and 6 have excellent adhesion to the diamond in ordersubsequently to form a good thermal compression bond; however, it isfound that the use of the chromium and gold layers on diamond with thenovel thermal compression bonding procedure yet to be described improvesthestrength of the chromium-diamond bond. When the bonding pressure hasbeen applied, it is found that the adhesion of the evaporatedchromiumfilms 3 and 6 to the diamond is thereby increased considcrably.The method of coating the diamond and thermal compression bonding hasproduced mechanically strong bonds where breaking forces are realized ashigh as 20,000 pounds per square inch for gold-togold bonds. It will berecognized by those skilled in the art that conventional vacuumsputtering or evaporation methods may be used to deposit layers 2, 3, 6,and 7.

Heat sink 1 is prepared for thermal compression bonding by theapplication of a similar thin layer 17 of chromium, followed by theapplication of a gold layer 16. in this discussion, the term thermalcompression bonding is taken to mean a process for fabricating'a robustpermanent bond between two metal surfaces, simultaneously using heat andpressure without melting either metal surface. The bond which results isformed by solid state diffusion, for example, of atoms from gold layer 2into the gold surface 16 of copper plate 1 and vice versa under veryhigh pressure and at a moderately elevated temperature, as will befurther described. Suitable bonds may be also made between gold orsilver layers or one layer may be silver and the other gold. Metals arepreferred that have high electrical and thermal conductivity.

To facilitate thermal compression bonding of gold layer 2 to the goldsurface of heat sink 1, a preformed planar structure 10 having agenerally centrally located aperture and made by a conventionalphotolithographic method is bonded directly to gold layer 7. The bondbetween gold layer 2 and gold layer 16 on the copper heat sink 1 andthat between gold layer 7 and the preformed structure 10 are madesimultaneously, as illustrated in FIG. 2. The procedure is to place thediamond heat conducting element 4 with its bonding layers 2 and 3 on thesurface of gold layer 16 of the copper heat sink 1. Next, the preformedelement 10 is positioned on the gold layer 7 so that the aperture 15exposes the region to which diode 11 is to be affixed. A flat bondingtip 20 is lowered into intimate contact with the preformed structure 10and, using a conventional mechanical or other press, a pressure of theorder of 20,000 pounds per square inch is brought to bear upon the uppersurface of the preformed element 10. During this action, the assembly ismaintained at an elevated temperature, typically about 250 to 350Centigrade. The simultaneous pressure and heating generates very strongbonds simultaneously between the preformed element 10 and gold layer 7and between the opposite gold layer 2 and gold layer 16 of the copperheat sink 1. The pressure will generally be sufficient to cause diamond4 and metal layers 2, 3 to indent the surface of heat sink 1, asgenerally shown in FIG. 1.

Details of the mechanical press used in the bonding step need not besupplied here, since commercially available hydraulic or other presses,equipped with standard force gauging or control instruments, areadequate for the purpose. When the thermal-compression bonding processis carried out according to the novel method, bonding pressures asrepresented by arrow 21 as high as 20,000 pounds per square inch may beapplied successfully without any fear of damaging the semiconductordiode 11 or the surface to which it is to be affixed. Highly reliableand uniform thermal comprssion bonds with minimum risk to both deviceand quality of the bond can then be accomplished at relatively lowpressures. The desired gold layer thermal bonding temperature (275 to350 Centigrade) is supplied by placing the diode device within aconventional heater of the type known in the art as a heat column, sothat heat flows into heat sink 1 and diamond 4 in the sense of arrow 28and thus to the junctions to be bonded. Automatically controlled heatersmay be employed which convenitonally control the temperature at thedesired junctions so that they lie in the range from 300 to 320Centigrade, for example, thus ensuring that high quality bonds areregularly formed.

The structure is further completed, after the preformed element 10 isbonded in place, by attaching the semiconductor diode 11 to the goldlayer 7. As seen in the FIG. 1, diode ll is supplied with a thinchromium layer 9 and an extenral gold layer 8. The respective gold andchromium layers 8 and 9 have been formed on the side of the diode llclosest to the active diode junction 11a. Beneficially, the junction 11ais placed as close possible to the diamond heat conducting element 4, sothat the flow of heat generated in junction 11a into the diamond element4 and out of the latter into heat sink 1 is enhanced.

For this purpose, a polished surface of diode 11 is supplied with alayer 9 of chromium about 300 Angstrom units thick and a layer 8 of goldabout 1,000 Angstrom units thick. To install diode 11, it is placed inposition and a conventional method such as a thermal compression bondingmethod is used to bond the gold layers 7 and 8. Non-conventional methodsof bonding diode 11 to the diamond heat conducting element 4 may also beused, especially in the instance of diode elements having irregularshapes. Particularly, if

the active diode region is to be ring shaped, two ring- V shaped bondswill be made between the aforementioned gold layers, and such may beaccomplished by employing the methods described in the H. Kroger. C. N.Potter US. Pat. application Ser. No. 222,771, filed Feb. 2, 1972, issuedas US. Pat. No. 3,761,783, Sept. 25, i973 for a High Frequency Diode andMethod of Manufacture and assigned to the Sperry Rand Corporation.

To complete the structure of the novel diode, a gold strap 13 isfastened by a conventional thermal compression or other bonding methodto an exposed surface of the preformed element and also to an exposedgold surface layer 16 of heat sink 1. The conductive strap 13beneficially serves to carry microwave energy from the preformed element10 to the normally electrically grounded heat sink 1. Use of the strap13 in conjunction with the preformed structure 10 eliminates the priorart requirements for applying a thin layer of conductive metal to theirregularly shaped sides 5 of the diamond element 4. A bias lead 12which also assists in coupling the high frequency electric power acrossdiode 11 is finally affixed to a surface of diode ll opposite thepreformed element 10.

It is seen that, according to the invention, a highly efficient heatpath is provided between the active semiconductor junction 11a and theheat sink 1. It will be understood that a very efficient path for heatflow from the heat sink 1 to external means for dissipating such heat,such as cooling fins or other fluid cooling elements, may be readilyprovided as indicated at 24 in FIG. 2. By such an arrangement, thetemperature of heat sink 1 may be readily held near ambient temperature,as is desired. While maximizing the rate of flow f0 heat away from thediode active junction, the novel configuration and method also minimizesmicrowave frequency losses.

Use of the preformed structure 10 allows thermal compression bonding ofa desirable, very thin metallized diamond heat conductor 4 to a heatsink 1 without damage of any kind to the thin metal layers 6 and 7,especially in the area where the thinly metallized semiconductor diode11 is to be bonded. Because the area where the semiconductor diode 11 isto be bonded is protected during the heat sink bonding step, it ispossible to use much thinner than conventional layers 8, 9 of bondingmetal between the semiconductor diode 11 and the diamond element 4, andthus to achieve lower than conventional values of series thermalresistance for these elements. There is a significant economy offabrication steps according to the novel method in that the preformedstructure 10 and the thermal sink 1 are bonded to the diamond element 4simultaneously. The preformed structure 10, generally of the shapeshown, may be fabricated by conventional photolithographic techniques sothat it may be placed in very close proximity to the periphery of thesemiconductor diode 11, thus minimizing the series electrical resistanceat microwave frequencies between the semiconductor diode ll and thepreformed element 10 through the surface of the thin metal layer 7. Thepreformed element 10 it-.

self serves as a low loss path to the signals of microwave frequencies,having a thickness several times that of the skin depth. The preformedelement also serves as a bonding base for the strap 13 which carriesmicrowave power from the preformed structure 10 to the heat sink 1.Additionally, use of the strap 13 in conjunction with preformedstructure 10 eliminates the need of mctallizing the irregularly shapedsides 5 of the diamond 4 with a metal layer of thickness greater thanthe skin-depth at the high frequencies involved.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departure from thetruescope and spirit of the invention in its broader aspects.

1 claim:

1. In a high frequency semiconductor device,

diamond heat conductor means having first and second opposedsubstantially parallel flat polished sur faces,

first and second high' electrical and thermal conductivity metal layersbonded separately to said respective first and second opposedsubstantially parallel flat polished surfaces,

massive heat sink means bonded to said first metal layer,

preformed apertured plate means bonded to said second metal layersimultaneously with the bonding of said massive heat sink means to saidfirst metal layer, and

active semiconductor means bonded substantially concentrically withinsaid aperture to said second metal layer within said aperture.

2. Apparatus as described in claim 1, wherein said diamond heatconductor means comprises type lla diamond material.

3. Apparatus as described in claim 2 wherein said first and second highelectrical and thermal conductivity metal layers each comprise;

a thin layer of chromium bonded to said diamond heat conductor means,and

a thin layer of gold bonded to said thin layer of chromium.

4. Apparatus as described in claim 1 further comprising high frequencyelectrical conductor means bonded to said preformed apertured platemeans and to said massive heat sink means.

5. Apparatus as described in claim 4 further comprising bias powerelectrical conductor means affixed to said semiconductor device oppositesaid second metal layer.

1. In a high frequency semiconductor device, diamond heat conductormeans having first and second opposed substantially parallel flatpolished surfaces, first and second high electrical and thermalconductivity metal layers bonded separately to said respective first andsecond opposed substantially parallel flat polished surfaces, massiveheat sink means bonded to said first metal layer, preformed aperturedplate means bonded to said second metal layer simultaneously with thebonding of said massive heat sink means to said first metal layer, andactive semiconductor means bonded substantially concentrically withinsaid aperture to said second metal layer within said aperture. 2.Apparatus as described in claim 1, wherein said diamond heat conductormeans comprises type IIa diamond material.
 3. Apparatus as described inclaim 2 wherein said first and second high electrical and thermalconductivity metal layers each comprise; a thin layer of chromium bondedto said diamond heat conductor means, and a thin layer of gold bonded tosaid thin layer of chromium.
 4. Apparatus as described in claim 1further comprising high frequency electrical conductor means bonded tosaid preformed apertured plate means and to said massive heat sinkmeans.
 5. Apparatus as described in clAim 4 further comprising biaspower electrical conductor means affixed to said semiconductor deviceopposite said second metal layer.